Modern passenger aircraft, such as the Boeing 767, include a passenger entertainment system (PES) for providing audio entertainment in the form of music, movie sound track, public address, etc. to the individual passenger seats of the aircraft. The seats are arranged in seat groups, each seat having its own acoustic type headset which is driven by a pair of audio transducers located within a passenger control unit (PCU) mounted in each seat. The transducers convert the electrical audio signals of the PES into acoustic waves which are communicated through the sound tubes of the headset to the ears of the passenger. Each PCU also includes channel selection and volume controls which allow the passenger to control the audio presented to the headset.
The PES systems are well known; the contemporary systems using a digitized audio signal distribution format, such as the pulse code modulation (PCM) format disclosed in U.S. Pat. Nos. 3,566,038 to Slavin and 3,795,771 to Gundersen, et al, with the PCM being decoded by a signal demultiplexer (DMUX) included within each seat electronics box (SEB) of each seat group. Typically, the PES is used in combination with a passenger service system (PSS) which provides the communication interface between the passenger and the available services, i.e. service calls to the attendant, or for turning on the individual seat lamps and air conditioning apparatus. The functions of the PES and PSS are autonomous, such that each is independent of the other in providing their respective passenger functions. The two systems do, however, share a common dependency with respect to the built-in test equipment (BITE) which performs an in situ test routine for determining the combined PES/PSS system integrity.
The PES/PSS BITE systems are also well known, as disclosed in U.S. Pat. No. 3,585,307 to Greensberg. The BITE tests a multitude of system functions, i.e. the operability of the various subsystems associated with the audio and service networks, one of which is the operability of the headset transducers. The BITE system of Greenberg provides for the transducer testing by mounting self-test circuitry within the PCU itself. The test circuitry includes band-pass filters inserted within the audio lines to the individual transducers from the seat group DMUX and separate transducer driver sense means, which in combination provide indications of: (1) the presence of an output from the DMUX audio driver, and (2) the electrical integrity of each transducer by sensing the presence of a prescribed transducer impedance value. This circuitry must be duplicated for each headset, such that each PCU includes dedicated transducer BITE components. The BITE transducer circuitry of Greenberg also requires individual filters and precision resistor networks for each transducer to permit accurate sensing of the selected transducer impedance value, all of which results in higher cost, higher weight, and lower BITE reliability.